Freestanding nanostructures have many applications in the fields of photonics and mechanics. For photonic devices, refractive index contrast is needed to confine an optical mode. For silicon devices, technologies such as silicon-on-insulator (SOI) can be utilized to achieve refractive index contrast where a high quality silicon device layer is supported by a low index substrate. For nanomechanical resonators and oscillators, mechanical freedom for motion is also desirable. Undercutting a supporting substrate using selective etching can create freestanding nanostructures for thin film on insulator technologies. However, for several materials such as diamond, lithium niobate (LiNbO3), silicon carbide (SiC), and gallium nitride (GaN), to name a few, high quality thin film heterolayers are not readily available. Wafer-scale polycrystalline thin films are available for several materials (diamond in particular) where thin films are grown directly on disparate substrates. However these films tend to have inferior properties, both optically and mechanically, due to grain boundaries, surface roughness, and inherent film stress.
An alternative approach to realize nanoscale photonic and mechanical devices is to fabricate devices starting from the bulk material. Typical fabrication of such structures employs a technique called reactive ion etching (RIE). In RIE, a plasma is ignited in a chamber using strong radio frequency (RF) electromagnetic fields. The oscillating field strips the process gas of its electrons creating a plasma. A direct current (DC) bias is then established at a sample (target) and as a result of the voltage difference, ions are accelerated towards the sample chemically and mechanically etching the sample. Techniques exist whereby modulating the etch chemistry by varying RF power or gas pressure can create undercuts to produce freestanding structures. However such techniques are difficult to reproduce consistent device cross sections and limit the final geometry one can fabricate.
Another technique utilizes placing a sample within a Faraday cage placed inside the RIE chamber where the cage electrically isolates the interior of the cage from the electromagnetic field produced by the RF generators. The Faraday cage typically consists of a fine metal mesh where ions are still physically able to pass through, however the field within the metal mesh is drastically attenuated and altered. Therefore ions incident on the cage are accelerated along a path perpendicular to the Faraday cage and ultimately reach the sample of interest to perform the etching function. However due to the physical size of the Faraday cage inside the RIE chamber, the uniformity of the incident ions drastically vary even along short distances (i.e. several 10s of microns). This leads to difficultly in producing identical devices along a sample of several millimeters. Therefore, due to the physical size of the Faraday cage, it's not feasible to perform this type of etching on a wafer scale. Other techniques to achieve similar devices include crystal ion-slicing or focused ion beam milling. In both cases there is appreciable ion damage to the device layer, which again significantly limits the performance of the resulting devices. Therefore, there is a need in the industry to address one or more of the above mentioned issues.